This invention relates to a polarization mode dispersion compensating apparatus, and more particularly to an apparatus for compensating polarization mode dispersion that may occur to signal light in an optical transmission line.
Along with the spread of the Internet, a demand for larger transmission capacity has been increasing without stopping. As is known in the art, optical fiber communication is suitable for the large capacity transmission. In the optical fiber transmission, it is relatively easy to increase the transmission capacity by employing a wavelength multiplexing transmission system, in which signal lights with different wavelengths propagate on only one optical fiber, so as to increase the number of the wavelengths and accelerate the modulation rate of the signal lights with the respective wavelengths. It has been the mainstream to employ a return to zero (RZ) signal in order to improve the receiving sensitivity and decrease cross phase modulation when the wavelength multiplexing transmission is performed.
Ideally speaking, an optical fiber should be rotationally symmetric about a center axis of a core (i.e. a center axis of the fiber). However, owing to the slight asymmetricity due to fluctuations in the production process, the amount of chromatic dispersion of the signal light differs according to an azimuthal direction around the center axis of the core. This causes the so-called polarization mode dispersion. When the modulation rate of the signal light exceeds 5 Gbit/s, as shown in FIG. 8, the RZ signal is divided into two orthogonal polarization components (i.e. the so-called TE and TM components) in the time domain due to the polarization mode dispersion. This causes bit errors in a receiving process at a receiving side. Although the time intervals of the divided orthogonal components depend on the condition of the optical transmission line, they generally vary disorderly with time.
Means for compensating such polarization mode dispersion are disclosed by Fabian Roy et al. in OFC""99IOOC (OFC (Optical Fiber Communication) and the International Conference on Integrated Optics and Optical Fiber Communications (IOOC)), TuS 4-1, pp. 275-278 and by Hiroki Ooi et al. in OFC""99IOOC (OFC (Optical Fiber Communication) and the International Conference on Integrated Optics and Optical Fiber Communications (IOOC) WE 5-1).
A conventional polarization mode dispersion compensating apparatus generally comprises a polarization controller for converting a signal light from an optical transmission line into two orthogonal polarizations, a polarization mode dispersion compensating element for giving a certain time difference between the two orthogonal polarization components of the output light, and a measurer for measuring the intensity or degree of polarization (DOP) of an output light from a polarization mode dispersion compensating element and for controlling the polarization controlling amount or rotational angle of the polarization controller so as to maximize the measured result. In the former reference, the DOP is measured. In the latter reference, a clock component of 20 GHz is measured which frequency is half of a 40 Gbit/s NRZ signal light.
The polarization controller comprises a configuration in which a quarter wave plate and a half wave plate are connected in serial, and the measurer rotates mechanically both wave plates about the optical axis according to the measured result. The polarization of the incident light is, as a result, converted into a linear polarization. The polarization dispersion compensating element generally comprises a polarization maintaining fiber. The polarization maintaining fiber comprises mutually orthogonal slow and fast axes having different chromatic dispersions each other. That is, since the propagation speeds of the signal light differ between the two axes, the polarization maintaining fiber can give the suitable amount of the polarization mode dispersion according to the difference of propagation speeds between the two axes and the propagation length. In the conventional art, the polarization controller is feedback-controlled so as to maximize the optical intensity or DOP of the output light from the polarization maintaining fiber. In this way, the time difference between the orthogonal components given at the optical transmission line is removed by the polarization maintaining fiber and thus the polarization mode dispersion is compensated.
In the standard long-haul optical fiber transmission line, the polarization of the signal light varies every several ten msec at the shortest. However, the response of the mechanical polarization controller having performed per second, the existing polarization mode dispersion compensating apparatus is unable to follow the fast polarization variation.
Also, in the conventional system, the mechanical polarization converter is employed and thus it is difficult to use it over a long period. In other words, it is not very reliable.
Moreover, in the conventional apparatus, the polarization maintaining fiber is employed of having a constant compensation amount for the polarization mode dispersion. Owing to this, when a signal light with a little amount of the polarization mode dispersion is input, the polarization maintaining fiber adds the polarization mode dispersion to the signal instead, conversely increasing the bit error rate.
An object of the present invention is to provide a polarization mode dispersion compensating apparatus for adapting to any polarization state of input signal light and compensating the polarization mode dispersion.
Another object of the present invention is to provide a polarization mode dispersion compensating apparatus for compensating the polarization mode dispersion of a wider range.
A further object of the present invention is to solve the foregoing inconveniences and to provide a polarization mode dispersion compensating apparatus for responding more rapidly.
A still further object of the present invention is to provide a polarization mode dispersion compensating apparatus for automatically adapting to a polarization state of input signal light and compensating the polarization mode dispersion.
An even further object of the present invention is to provide a polarization mode dispersion compensating apparatus for maintaining high reliability over a long period.
According to the invention, an apparatus for compensating a polarization mode dispersion of an input signal light comprises a polarization converter for converting the polarization of the input signal light into a linear polarization, a polarization extractor for extracting at least one polarization component of two mutually orthogonal components in an output light of the polarization converter, a signal extractor for extracting a signal of a predetermined component from an output light of the polarization extractor, and a controller for controlling the polarization converter so as to increase the output of the signal extractor according to the output of the signal extractor.
With the aforementioned configuration, in the invention, the polarization mode dispersion compensating apparatus automatically adapts to the polarization state of the input signal light and compensates the polarization mode dispersion of the input signal light.
The signal extractor preferably comprises a photodetector for converting the output light of one polarization from the polarization extractor into an electric signal and an extractor for extracting the signal of the predetermined component from the output of the photodetector and applying it to the controller. The extractor comprises either electric filter for extracting the intensity of the clock component of the input signal light or for extracting a mean optical intensity of the input signal light. In this structure, the polarization mode dispersion of the input signal light can be compensated with such simple configuration.
The signal extractor preferably comprises a first photodetector for converting the output light of one polarization of the polarization extractor into an electric signal, a first extractor for extracting a signal of the predetermined component from an output of the first photodetector, a second photodetector for converting an output light of the other polarization of the polarization extractor into an electric signal, a second extractor for extracting a signal of the predetermined component from the output of the second photodetector, a comparator for comparing the outputs of the first and second extractors, and a selector for selecting one of the outputs between the first and second extractors and applying it to the controller according to the compared result of the comparator. The compensating apparatus further comprises a signal selector for selecting a signal to be carried on either one of polarizations from the polarization extractor according to the compared result of the comparator. The first and second extractors respectively comprise either electric filter for extracting the intensity of the clock component of the input signal light or for extracting a mean optical intensity of the input signal light. In this structure, even when the principal axis is changed, the polarization mode dispersion of the input signal light is continuously compensated without any difficulty.
The polarization converter comprises an apparatus for rotating the polarization of the input signal light with Faraday rotation. The polarization converter preferably comprises a first converter for moving the polarization of the input signal light along a parallel of latitude on a Poincare sphere using Faraday rotation, a wave plate for moving the output light of the first converter onto the equator of the Poincare sphere, and a second converter for moving the polarization of the output light of the wave plate along the equator of the Poincare sphere. The first and second converters respectively comprise a Faraday element, a magnet generator for applying a magnetic field in a direction of the optical axis of the Faraday element to the Faraday element according to a driving current from the controller, and a magnet for applying a magnetic field, which is in a direction orthogonal to the optical axis of the Faraday element and has the steady intensity for magnetically saturating the Faraday element, to the Faraday element. In this structure, it is possible to convert the polarization without moving parts and thus the high reliability is obtained over a long period. Also, a high-speed response is realized.
According to the invention, an apparatus for compensating the polarization mode dispersion of the input signal light comprises an optical divider for dividing the input signal light into two portions, first and second dispersion compensators, a signal selecting switch and a switch controller. The first dispersion compensator has a first polarization converter for converting a polarization of one output light from the optical divider into a linear polarization, a first polarization extractor for extracting a predetermined polarization component from the output light of the first polarization converter, and a first controller for controlling the polarization conversion of the first polarization converter so as to increase the intensity of the output light from the first polarization extractor. The second dispersion compensator has a second polarization converter for converting a polarization of the other output light from the optical divider into a linear polarization, a second polarization extractor for extracting a predetermined polarization component from the output light of the second polarization converter, and a second controller for controlling the polarization conversion of the second polarization converter so as to increase the intensity of the output light from the second polarization extractor in such condition that the control signal for the second polarization converter is restricted within a predetermined restricted region. The signal selecting switch selects either one of the outputs from the first and second dispersion compensators and at first selects the output of the first dispersion compensator. The switch controller monitors the controlled conditions of the first and second polarization converters by the first and second controller and controls the first and second controller as well as the signal selecting switch according to the monitored result. When the control signal of the first controller for the first polarization converter exceeds the restricted region, the switch controller controls the signal selecting switch to select the output of the second dispersion compensator and directs the second controller to control the polarization conversion of the second polarization converter so as to increase the output light from the second polarization extractor regardless of the restricted region of the control signal for the second polarization converter.
With this configuration, it becomes possible to compensate the polarization mode dispersion adaptively to the polarization state of the input signal light. Also, when the polarization of the optical transmission line changes to such degree that turns around the Poincare sphere more than once, the second dispersion compensator is immediately selected in order to prevent the polarization converter from continuously receiving the excessive control signal and depending on the dispersion compensating condition with the excessive control signal. This configuration hence obtains the high reliability.
Preferably, when the switch controller regulates the second controller to control the polarization conversion of the second polarization converter so as to increase the output light of the second polarization extractor regardless of the restricted region of the control signal for the second polarization converter, the switch controller adjusts the first controller to control the polarization conversion of the first polarization converter so as to increase the output light of the first polarization extractor in such condition that the control signal for the first polarization converter is restricted within a predetermined restricted region. In this structure, when the second dispersion compensator receives the excessive control signal, the switch controller can immediately switch to the first dispersion compensator again. The stable dispersion compensation therefore is realized continuously over a long period.
The first and second controllers respectively control the polarization conversions of the first and second polarization converters so as to increase signals of predetermined component obtained from the predetermined polarization components extracted by the first and second polarization extractors. The signal of the predetermined component comprises for instance a signal showing the clock component intensity of the input signal light.
Preferably, the first controller further comprises a first photodetector for converting the output light of one polarization out of the two orthogonal polarization components from the first polarization extractor into an electric signal, a first signal extractor for extracting a signal of the predetermined component from the output of the first photodetector, a second photodetector for converting the output light of the other polarization from the first polarization extractor into an electric signal, a second signal extractor for extracting a signal of the predetermined component from the output of the second photodetector, a first comparator for comparing the outputs from the first and second signal extractors, and a first selector for selecting one of the outputs from the first and second signal extractors according to the compared result of the first comparator, and controls the polarization conversion of the first polarization converter so as to increase the output of the first selector. The second controller further comprises a third photodetector for converting the output light of one polarization of the two orthogonal polarization components from the second polarization extractor, a third signal extractor for extracting a signal of the predetermined component from the output of the third photodetector, a fourth photodetector for converting the output light of the other polarization from the second polarization extractor into an electric signal, a fourth signal extractor for extracting a signal of the predetermined component from the output of the fourth photodetector, a second comparator for comparing the outputs from the third and fourth signal extractors, a second selector for selecting one of the outputs from the third and fourth signal extractors according to the compared result of the second comparator, and controls the polarization conversion of the second polarization converter so as to increase the output of the second selector. Also, the first dispersion compensator further comprises a first signal selector for selecting a signal to be carried on either one of the polarizations from the first polarization extractor according to the compared result of the first comparator, and the second dispersion compensator further comprises a second signal selector for selecting a signal to be carried on either one of the polarizations from the second polarization extractor according to the compared result of the second comparator. In this structure, even if the principal axis is changed, the polarization mode dispersion is constantly compensated without difficulty.
Preferably, the first and second polarization converters comprise apparatuses for rotating the polarization of the input light with Faraday rotation. To put it more concretely, the first and second polarization converters respectively comprise a first converter for moving the polarization of the input signal light along a parallel of latitude on the Poincare sphere using Faraday rotation, a wave plate for moving the output light of the first converter to the equator of the Poincare sphere, and a second converter for moving the polarization of the output light of the wave plate along the equator of the Poincare sphere. The first and second converters respectively comprise a Faraday element, a magnet generator for applying a magnetic field in the direction of the optical axis of the Faraday element to the Faraday element according to driving currents from the first and second controllers, and a magnet for applying the magnetic field, which is in a direction orthogonal to the optical axis of the Faraday element and has the steady intensity for magnetically saturating the Faraday element, to the Faraday element. In this structure, it is possible to convert the polarization without moving parts and thus the high reliability is obtained over a long period. Also, a high-speed response is realized.